Assay test kits currently are available for testing a wide variety of medical and environmental conditions or compounds, such as a hormone, a metabolite, a toxin, or a pathogen-derived antigen. Most commonly these tests are used for medical diagnostics either for home testing, point of care testing, or laboratory use. For example, lateral flow tests are a form of immunoassay in which the test sample flows along a solid substrate via capillary action. Some tests are designed to make a quantitative determination, but in many circumstances all that is required is a positive/negative qualitative indication. Examples of such qualitative assays include blood typing, most types of urinalysis, pregnancy tests, and AIDS tests. For these tests, a visually observable indicator such as the presence of agglutination or a color change is preferred.
In the field of immunoassay detection, the presence of an antigen may be inferred by the presence of specially created antibodies which contain reflectance or fluorescence labeling materials. The fluorescence material may be excited with a light pulse, and the fluorescence decay may be detected soon after the excitation source has been turned off. The period during this after-glow yields the best signal-to-noise ratio of the immunoassay labels, since the excitation source is turned off, thereby eliminating any background signal. To further improve the sensitivity, a method of detection known as time resolved fluorescence detection may be employed (see, e.g., FIG. 1). Generally, the labeled material 25 may be illuminated with a pulsed light source, such as with a laser or LED 20. After the light source has been turned off, the amount of fluorescence after-glow may then be detected with a light detector, such as a PIN photodiode 30, and its signal amplified with a transimpedance amplifier (TIA) 40. With further signal amplification 50, the signal may be applied to a Sample/Hold Circuit 60, where the output is further averaged usually by filtering 70. The final output 75 may then be measured with an Analog-to-Digital Converter (ADC) 80, and further processed with a micro-controller 90 to deliver the signal to a user-friendly output, such as an LCD display 100.